1. Field of the Invention
The present invention relates to an adhesive film which comprises at least two continuous polymer phases A and B each formed from a polymer or polymer mixture A (polymer A for short) and a polymer or polymer mixture B (polymer B for short) and which is obtainable by filming at least one of the polymers, A or B, initially in the form of an aqueous polymer dispersion.
2. Discussion of the Background
With pressure-sensitive adhesives (PSAs) there is a desire not only for effective adhesion to the substrate but also for a sufficient internal strength within the adhesive layer (cohesion). Adhesion and cohesion constitute divergent performance properties. Measures which bring about an improvement in the adhesion generally lead at the same time to a deterioration in the cohesion, and vice versa.
PSAs based on aqueous polymer dispersions which are obtainable by emulsion polymerization have been known for a long time. The polymers in such dispersions are, in particular, polyacrylates or vinyl acetate/ethylene copolymers. With emulsion polymers of this kind an improvement in the adhesion and the cohesion has to date been achieved by selection of appropriate comonomers or additives. By way of example, reference may be made to DE-A 103 23 048, whereby positive effects are achieved by adding silicon compounds to the polymer dispersion. EP-A 625 557 discloses PSAs which comprise hydroxyalkyl acrylates.
There is a desire for alternative methods of enhancing the adhesion and cohesion of PSAs. The methods ought to be extremely simple and the improvements in performance properties ought to be achieved in particular without necessarily having to use new or expensive raw materials.
Polymer films comprising at least two continuously intermixed polymer phases are known per se. Bicontinuous polymer phases are described for example by S. Kirsch, A. Pfau, E. Hädicke and J. Leuninger in Progress in Organic Coatings 45 (2002), 193-204.
U.S. Pat. No. 5,670,557 discloses PSA having a continuous polymer phase. The preparation process described in U.S. Pat. No. 5,670,557 uses aqueous microemulsions of monomers. The emulsified particles comprise the hydrophobic monomers; hydrophilic monomers are in solution in water. The microemulsion is coated onto the desired backing, and only after that is the polymerization carried out by irradiation with high-energy light directly on the backing.
The PSAs obtained are said to be suitable in particular for applications requiring electrical conductivity on the part of the PSA.
U.S. Pat. No. 5,356,993 describes the production of phase-separated adhesives, seals, and coatings by coating onto a suitable backing with subsequent irradiation. The system is composed of an epoxidized diene polymer and of a further epoxy polymer. Polymer dispersions are not mentioned.